This invention relates generally to the use of optical excitation, particularly induced by a tunable laser, for the detection of new compositions and improved methods of depositing a material like a photoconductive film on a substrate.
Various deposition processes like glow-discharge and chemical vapor deposition have been used for producing films and coatings for application in many areas, e.g. photovoltaics, microelectronics, electroptics, and superconductors. Despite the development of these processes as a sophisticated technological art, the fundamental physical and chemical processes involved are still not fully understood.
The lack of detailed information is understandable because the deposition environment is quite difficult to study analytically. Over the years a number of in situ analytical techniques have been applied to the deposition environment. These techniques, however, have a number of limitations which prevents a detailed understanding of the deposition process. For example, mass spectroscopy and vapor chromotography both require physical probes and/or remote sensing apparatus and therefore may not record the true distribution of chemical species inside a deposition reactor. Mass spectroscopy also relies upon known fragmentation patterns of calibrated molecules. Often these patterns overlap and therefore obscure the patterns of intermediate or transient products of the reaction taking place in the deposition reactor.
Other examples of analytical techniques are optical absorption and emission spectroscopy. Optical absorption spectroscopy lacks spacial resolution in three dimensions and is much less sensitive than laser-induced fluorescence, precluding its use for the detection of species present in low concentrations or in the vicinity of the important gassurface interface region. Emission spectroscopy by its very nature can only detect species that are in vibrationally or electronically excited states. The concentrations of species in the ground state far exceed those in the excited state and thus the method becomes effective only in monitoring a small fraction of the transient species present in the deposition process. Also, the excitation rate from the ground state to the observed excited state is often unknown and may depend on external parameters such as gas pressure and flow rate. Another limitation of these analytical techniques is that they do not provide a non-perturbing pinpoint probe that can accurately measure low, i.e. sub-Torr, concentrations of highly reactive chemical species in the presence of rapidly changing chemical and temperature fields.
There have been particular problems in measuring the energy distributions of particles in glow discharges, thus complicating the monitoring of the plasma used in photovoltaic film manufacturing. Reaction rates, available product channels, and transport phenomena all depend upon the partitioning of energy in the discharge. Because of the nonequilibrium nature of glow discharges, however, the distribution of energy among different species and among different degrees of freedom cannot be characterized simply by one temperature. The extent to which different temperatures are needed for each degree of freedom and for each species is not known completely. Also, not every discharge system used in photovoltaic processing provides a convenient, optically emitting species from which a temperature measurement can be made.
A variety of transient atomic and molecular species are produced in chemical vapor deposition, diffusion flame, and glow-discharge reactors by pyrolysis, chemical reactions, or electron bombardment. Some of these species may play a prominent role in the deposition of the film itself. Monitoring their concentrations accurately and with a nondestructive analytical technique would be very helpful in understanding the fabrication of the films. It is even more difficult to monitor the existence of transient species which may affect the quality of the film or coating being deposited if the species itself is not optically emitting or its emissions are not resolvable by the techniques described above.
In particular, photovoltaic quality films of amorphous SiH.sub.x Fy prepared by the plasma-induced reaction of, for example, SiF.sub.4 and H.sub.2, appear to contain unusual bonding configurations involving the intimate association of silicon with both fluorine and hydrogen atoms. The creation of radical sub-fluorides of silicon has been suggested as a mechanism for the deposition of such films. The presence of various excited-state neutral silicon radical species, i.e. SiF, SiH, SiF.sub.3, and Si has been demonstrated through the use of optical emission techniques in diffusion flames. Other radicals, especially in the ground state, have escaped detection and thus have hindered analysis of the effect these radical species have on the quality of the deposited film.